It is known that a loudspeaker having a flat frequency characteristic (i.e. a characteristic in which the sound pressure level P is given as a function of the frequency with a constant input voltage on the loudspeaker), when mounted in a car, yields a non-flat frequency response, which is undesirable.
This problem is set forth in John Carter's publication entitled "Digital simulator for automotive sound system design", Audio Systems P-142, page 31, published by the Society of Automotive Engineers, February 1984.
FIG. 1 in the above publication gives an example of such a frequency response characteristic in a car. The curve shown therein exhibits a hump at frequencies between approximately 200 and 400 Hz.
In order to realise nevertheless a flat frequency response characteristic in the car, said publication proposes the use of a frequency equalizer which must therefore be adjusted to provide frequency response characteristic which is substantially the inverse of the frequency response characteristic of the loudspeaker in the car.
The publication "Acoustic characteristics of the vehicle environment" by L. Klapproth, preprint No. 2185 (C-3) of the 77th AES Convention in Hamburg of March 1985, shows response characteristics measured in different types of cars and measured at different locations in a car. The author states that the sound in the test cars is boosted by approximately 10 dB in the frequency range from approximately 100 to 400 Hz. This means that in general the frequency equalizer must have a frequency characteristic which exhibits a broadened dip of 5 to 10 dB in the frequency range having a lower limit-frequency lying somewhere between 100 and 200 Hz and having an upper limit frequency lying somewhere between 400 and 500 Hz.
It is an object of the invention to provide a loudspeaker having a frequency characteristic which inherently exhibits a (broadened) dip. Moreover, it is an object of the invention to propose additional steps enabling the loudspeaker to be dimensioned in such a way that the dip has the desired depth of 5 to 10 dB and is situated in a desired frequency range (measured in an anechoic room) so that a separate equalizer is not needed in order to obtain a flat frequency response characteristic in the car. It is a further object to provide a loudspeaker having a broadened dip (preferably 5-10 db) in the low frequency range of its frequency characteristic, e.g. in the frequency range of 100 Hz to 500 Hz.
The invention is applied to a loudspeaker of a construction as disclosed in German Pat. Specification No. 3,123,098. This known transducer comprises a diaphragm, a chassis, a magnet system coupled to the chassis, and a voice coil device coupled to the diaphragm, The voice-coil device is situated in an air gap defined by the magnet system, The diaphragm comprises a central part and a surrounding peripheral part which is coupled to the chassis along its outer circumference, the stiffness of the central part being higher than that of the peripheral part, and the voice-coil device being coupled to the central part, the ratio S.sub.2 /S.sub.1 complying with: EQU x.sub.1 .ltoreq.S.sub.2 /S.sub.1
where x.sub.1 is a specific value and S.sub.1 and S.sub.2 are the surface areas of the central part and the peripheral part respectively. However, the known transducer is intended to provide a flat frequency characteristic.
In a previously filed copending U.S. patent application No. 872,057, filed June 6, 1986, a loudspeaker of the same construction is described in which the ratio S.sub.2 /S.sub.1 complies with: EQU x.sub.1 .ltoreq.S.sub.2 /S.sub.1 .ltoreq.x.sub.2,
where x.sub.1 and X.sub.2 are a specific first and second value respectively, and S.sub.1 and S.sub.2 are the surface areas of the central part and the peripheral part respectively, and in which the ratio m.sub.2 /m.sub.1 complies with: EQU x.sub.3 .ltoreq.m.sub.2 /m.sub.1 .ltoreq.x.sub.4,
where x.sub.3 and x.sub.4 are a specific third and a specific fourth value respectively, and m.sub.1 is the mass of the central part and the voice-coil device, and m.sub.2 is the mass of the peripheral part. This loudspeaker is also intended to provide a flat frequency-response characteristic.